1. Field of the Invention
The present invention relates to an optical pickup control apparatus, an optical pickup control method, and an optical disc apparatus and, more particularly, to an optical pickup control apparatus and optical pickup control method suitably applied to an optical disc apparatus in which there is a need to correct the spherical aberration.
2. Description of the Related Art
An increase in the storage density of an optical disc has been demanded. In order to increase the storage density of an optical disc, a method of increasing the numerical aperture (NA) of an objective lens (increase in aperture ratio) or method of shortening the wavelength of a light beam can be considered.
For example, in an optical disc apparatus compliant with Blu-ray Disc (Registered Trademark) specification, a blue-violet laser having a wavelength of 405 nm and an objective lens having a numerical aperture of 0.85 are used to thereby achieve a surface recording density about five times that of a conventional Digital Versatile Disc (DVD).
Such an optical disc apparatus compliant with Blu-ray Disc (Registered Trademark) specification is configured to perform focus servo for focusing a laser beam on a disc recording surface and tracking servo for allowing a laser beam to trace the track on a disc surface. When it comes to the focus servo, in particular, application of an adequate focus bias to a focus loop realizes adequate servo movement of the objective lens.
Spherical aberration caused by an error in a base material for protecting the recording layer of an optical disc increases in proportion to the fourth power of the numerical aperture. Further, in a multilayer disc, spherical aberration varies in each recording surface. Thus, the optical disc apparatus compliant with Blu-ray Disc™ specification needs to certainly correct the spherical aberration and, therefore, is provided with a spherical aberration correction mechanism using an expander lens or liquid crystal element as an optical pickup.
Various adjustment methods for obtaining an adequate spherical aberration correction value and focus bias value based on the evaluation value such as the amplitude or jitter of an RF signal have been proposed for such an optical disc apparatus (refer to, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-95106).